1. Field of the Invention
The present invention relates to a display apparatus, and in particular, to a display apparatus including an organic electroluminescent (hereinafter, abbreviated as “EL”) element.
2. Description of the Related Art
As a light emitting element used for a display apparatus, a light emitting diode (hereinafter, abbreviated as “LED”) has been paid attention in recent years. With regard to drive methods of the light emitting element, a current drive method or a voltage drive method have been proposed. Hereinafter, taking the case of a current drive type organic EL element which is used as the light emitting element, a prior art light emission drive will be described.
The organic EL element is also referred to as an organic LED (OLED: Organic Light Emitting Diode), and it enables a plane-shaped spontaneous light emission in which high brightness light emission is possible, to be obtained. As described in C. W. Tang etc., “Organic electroluminescent diodes”, Applied Physics Letters, pages 913 to 915, volume 51, 1987, and C. W. Tang etc., “Electroluminescent of doped organic thin films”, Journal of Applied Physics, pages 3610 to 3616, volume 65, 1989, the EL element is arranged to emit light in high efficiency at low voltage, with organic layers which serve as light emitting layers, being stacked between a pair of electrodes (anode and cathode) depending on the function thereof, and by increasing the number of the stacked functional organic layers.
The basic element configuration of the organic EL element includes an EL light emission layer having an organic layer and hole transport layer between an anode and a cathode, and a stacked structure of anode/hole transport layer/EL light emission layer/cathode. Based on the element configuration, by adding an electron transport layer having an organic layer between the EL light emission layer and the cathode to form a stacked structure of anode/hole transport layer/EL light emission layer/electron transport layer/cathode, high efficiency has been devised.
Further, in some cases, in order to prevent carriers from passing through the EL light emission layer, a blocking layer is provided between the EL light emission layer and the electron transport layer, or, a metal thin film as an electron injection layer is provided between the cathode and the electron transport layer, so as to enable injection of the carriers to be performed under low voltage. With these, the improvement of the light emission efficiency has been devised.
In such a display element using the organic EL element as a light emitting element, by injecting holes and electrons into the light emission layer, light emission brightness is controlled. As drive methods for the display element, an active matrix type constant voltage drive or a constant current drive, is known, which has Thin Film Transistor (hereinafter abbreviated as “TFT”) such as that disclosed in Japanese Patent Application Laid-open No. 2001-147659. Since it emits light spontaneously by these drives, the organic EL element is going to be integrated to be high-density and used as a display element device. Moreover, by using organic EL elements emitting red (R), green (G) and blue (B) color, respectively, a full color thin film display can also be achieved.
With regard to image signal wirings used for the above-mentioned display element, as illustrated in FIG. 25, an image signal wiring 76 connected to R pixel, an image signal wiring 77 connected to G pixel, and an image signal wiring 78 connected to B pixel, are usually patterned so as to have a same width and thickness of a same material.
However, in a case of constant voltage drive type display element of Prior Art Embodiment, in order to cause the light emitting element to emit light between electrodes or through TFT by using constant voltage, charge time or discharge time of parasitic capacitance charges under current reduced due to the resistance of the image signal wirings from the source to the pixel circuit, will be required. If a scanning signal wiring falls into a non-selected state when the charge or the discharge of charges into or from the parasitic capacitance has not yet completed, light emission of the organic EL light emitting element will occur when the brightness which does not reach to a predetermined value. The charge-discharge time at this time will be shorter when the image signal voltage is low, and will be longer when the image signal voltage is high. Accordingly, as the light emission voltage of the constant voltage drive light emitting element is lower and the brightness thereof is higher, the image signal voltage will be smaller and the charge-discharge time will be shorter. Conversely, when a predetermined brightness cannot be obtained unless the light emission voltage of the light emitting element is high, the image signal voltage will be higher and the charge-discharge time will be longer.
A case where the display element is applied to a color-matrix display element having a plurality of light emitting elements corresponding to respective colors of R, G and B will be considered. In the case, when the light emission voltage of the light emitting element of B is low, and the light emission voltage of R is high, although depending on the white balance ratio thereof, in many cases, the image signal voltage of R will be higher and the image signal voltage of B will be lower. For this reason, as illustrated in FIG. 26, when stripes of a black color stripe (black stripe) 83 and a white color stripe (white stripe) 81 are displayed on the display element 1, at the upper part of the white stripe 81, blue color 80, an image signal voltage of which is low and a charge-discharge time of which is short, will begin to be displayed first (blue smear display). On the other hand, on the upper part of the black stripe 83, red color 82, an image signal voltage of which is low and a charge-discharge time of which is long, will be remained being displayed (red smear display).
Moreover, in a case of constant current drive type display element of Prior Art Embodiment, in order to cause the light emitting element to emit light between electrodes or through TFT by using constant current, charge time or discharge time of parasitic capacitance charges will be required until the compensation voltage due to the resistance of the image signal wirings is increased. If a hold signal wiring is in a non-selected state at a stage when the charge or the discharge of charges into or from the capacitance is not completed, light emission of the organic EL light emitting element will occur at a stage when the brightness thereof does not reach to a predetermined value. The charge-discharge time at that time will be shorter when the image signal current is large, and will be longer when the image signal voltage is small. Accordingly, when a predetermined brightness cannot be obtained unless the current efficiency of the constant current drive light emitting element is low and the light emission current thereof is large, the image signal current will be larger and the charge-discharge time will be shorter. Conversely, even if the current efficiency of the constant current drive light emitting element is high and the light emission current thereof is small, since the brightness is higher the image signal current will be smaller, the charge-discharge time will be longer.
A case where the display element is applied to a color-matrix display element having a plurality of light emitting elements corresponding to respective colors of R, G and B will be considered. In the case, when the light emission current of the light emitting element of B is large, and the light emission current of R is small, although depending on the white balance ratio thereof, in many cases, the image signal current of R will be smaller and the image signal current of B will be larger. For this reason, as illustrated in FIG. 26, when the stripes of the black stripe 83 and the white stripe 81 are displayed on the display element 1, on the upper part of the white stripe 81, blue color 80, an image signal current of which is large and a charge-discharge time of which is short, will begin to be displayed first. On the other hand, on the upper part of the black stripe 83, red color 82, an image signal current of which is small and a charge-discharge time of which is long, will be remained being displayed. As the countermeasure to this, in order to cause the charge-discharge time to be shorter, use of a low efficiency light emitting element that emits light at large current for all colors can be considered. However, in the case, there is a disadvantage of power consumption.
In relation to the above, Japanese Patent Application Laid-open No. 2003-255884 discusses a drive control apparatus equipped with a unit for compensating and controlling the brightness change due to a voltage drop occurs by the resistance part of wirings using a drive circuit. However, in the apparatus, in order to control these, a large-scale drive circuit has been required.